1. Field of the Invention
The present invention relates to a developing device for developing electrostatic images formed on an image carrying member by electrophotography or electrostatic recording methods, particularly used in photocopiers, printers, facsimile apparatuses, and so forth.
2. Description of the Related Art
Image formation apparatuses such as photocopiers and the like which use electrophotography cause a developing agent to adhere to an electrostatic image formed on an image carrying member such as photosensitive drum or the like. As for the developing agent, there are magnetic single-component developing agents, non-magnetic single-component developing agents, and two-component developing agents which have non-magnetic toner and a magnetic carrier. The different types of developing agents are used as appropriate.
FIG. 1 illustrates an example of a conventional developing device 1 suitable for adaptation according to the present invention using a two-component developing agent having non-magnetic toner and a magnetic carrier.
The developing device 1 shown in FIG. 1 has a developing container 2, and disposed inside the developing container 2 are two transporting screws 5 and 6 for stirring and transporting the developing agent, and two developing sleeves 8 and 9 disposed one upon the other, for developing an electrostatic latent image formed on an image carrying member 10.
In further detail, the developing device 1 has first and second developing sleeves 8 and 9 serving as developing agent carrying members disposed one upon another at an opening of the developing container 2 facing the photosensitive drum 10 serving as the image carrying member. A developing chamber 3 and a stirring chamber 4 separated by a partitioning wall 7 are formed at the far side from the opening of the developing container 2, with the developing chamber 3 formed above the stirring chamber 4. The first and second transporting screws 5 and 6 serving as the developing agent stirring and transporting means are disposed within the developing chamber 3 and stirring chamber 4, respectively.
The developing agent transported from the stirring chamber 4 to the developing chamber 3 is scooped up by the developing sleeve 8 by means of an N1 pole provided within a magnetic roller 8a which is magnetic field generating means provided in a non-rotating manner within the developing sleeve 8, and the rotation of the developing sleeve 8 bring the developing agent to a first developing area A where a developing magnetic pole S2 is situated, where the developing sleeve 8 and the photosensitive drum 10 face one another. Along the way, the layer of developing agent is subjected to restriction of thickness thereof by means of a developing agent restricting blade 11 which is a developing agent restricting member and a magnetic pole S1 facing the developing agent restricting blade 11 and acting in cooperation therewith. This forms a thin layer of developing agent, whereby the electrostatic latent image is developed at the first developing area A from the magnetic pole N3 situated downstream of the first developing area A in the direction of rotation of the developing sleeve 8 (first developing step).
Subsequently, the developing agent is handled to a magnetic pole S3 of a magnetic roller 9a which is magnetic field generating means provided in a non-rotating manner within the developing sleeve 9, and reaches a second developing area B where the developing sleeve 9 and the photosensitive drum 10 face one another (second developing step). The developing agent remaining at the second developing area B without being developed is transported into the developing container 2, and is recovered in the stirring chamber 4 at the lower portion of the developing container 2.
The characteristics of the above-described vertical-stirring twin-sleeve developing device 1 include the advantages in that:                (1) The size of the developing device 1 can be reduced due to the two transporting screws 5 and 6 being vertically disposed; and        (2) The number of times that developing can be performed is increased as compared with single-sleeve arrangements due to the developing sleeves 8 and 9 being provided, whereby developing efficiency increases, edge enhancement can be reduced, and further, the rotations of the developing sleeves 8 and 9 can be reduced.        
Now, the materials and configurations of the above-described developing sleeves 8 and 9 are selected as appropriate depending on the type of developing agent to be used. For example, in the event of using a two-component developing agent, a developing sleeve having magnetic field generating means such as a magnet or the like within is used, and primarily non-magnetic metals such as stainless steel or aluminum have conventionally been used as the material of the developing sleeve.
With a developing device 1 such as described above, the surface of the developing sleeve is subjected to surface-roughening processing, which improves the transporting capabilities of the sleeves transporting the two-component developing agent made up of toner and carrier to the developing areas, and also enables uniform coating of the developing agent on the developing sleeve.
Methods that have been proposed and implemented for roughening the surface of developing sleeves include sanding with sandpaper, beads blasting using spherical particles, sand blasting using indeterminate-form particles, combinations of the above methods, chemical etching using chemical processes, and so forth. However, conventional sleeves have had the following problems.
After developing sleeves of which the surface has been roughened by the above methods have been used for extended periods, a problem develops in that the toner or components in the toner tend to catch on and adhere to the coarse formations formed on the roughened surface. The toner adhering to the “valley” portions eventually fuse due to friction heat and the like, and consequently, the developing sleeve is stained. Further, in the event of using a two-component developing agent containing a carrier, the toner particles or particles of components in the toner tend to be pressed into the valley portions, and particularly the narrow valley portions of the coarse formations formed on the roughened surface, due to being pressed by the carriers, where they become embedded. The embedded toner particles eventually fuse due to extended use, and the sleeve surface tends to become stained by toner.
Further, the demand for increased image quality and increased speed, and demand for reduced electrical power consumption, in accordance with the increased demand for color photocopiers and the like, has led to smaller particles and lower softening points for the toner, so the tendency for the toner or toner components to fuse to the coarse formations formed on the roughened surface and lead to staining is becoming even stronger.
Once toner fuses to the surface of the developing sleeves, first, the image density begins to deteriorate since the amount of developing agent transported to the developing areas decreases. Also, a developing bias with DC voltage and/or AC voltage superimposed has been conventionally applied to the developing sleeves at the time of developing, in order to carryout suitable developing, but toner fused to the surface of the developing sleeve creates a high-resistance layer of the fused substance on the surface of the developing sleeve, and accordingly, a desired electrical field may not be formed at the developing area between the developing sleeve and the image carrying member at the time of developing. Consequently, sufficient developing effects of the developing bias cannot be obtained, leading to substandard images such as images with inferior image density or white spots.
Actual experimentation has shown this to be true. An experiment performed with a developing sleeve having staining comparable to that of a developing sleeve used for 10,000 copies yielded image density 0.2 lower than that of an unstained developing sleeve, and also yielded substandard images such as images with white spots.
Carrying this experiment out further with a twin-sleeve developing device showed that there was difference in the level of staining between the first developing sleeve 8 and the second developing sleeve 9.
The sleeve staining was measured by measuring the reflection of light off of developing sleeves before and after usage, using a reflective densitometer, with the optical density difference ΔD taken as the staining density. As a result, the staining level of the first developing sleeve 8 was 0.30, while the staining level of the second developing sleeve 9 was 0.15, showing that the staining of the first developing sleeve 8 was more severe.
Studying the reason for this in detail showed that a major factor in the staining level of the first developing sleeve 8 was the fact that the first developing sleeve 8 faces the developing agent restricting blade 11. This staining can be thought to be due to fusion of the toner owing to being pressed by the developing agent restricting blade 11.
Also, with the twin-sleeve developing method, the potential difference between electrostatic latent images is small in the first developing step with the first developing sleeve 8, and the final image quality is determined in the second developing step with the second developing sleeve 9, and so it has been found that developing capabilities depend on the developing efficiency of the first developing sleeve 8, and micro-level dot reproducibility, white spots, lop-sided distribution and like edge enhancement and the like, and other such macro-level image quality, strongly depends on the state of the magnetic brush of the second developing sleeve 9.
Accordingly, with the twin-sleeve developing method, it is important to prevent deterioration of developing due to staining of the first developing sleeve 8 as much as possible, and to prevent deterioration of the formation of the magnetic brush and transportation at the second developing sleeve 9.